Life is also possible in very special months
Liquid water is one of the most important conditions for the emergence of life as we know it on Earth. Now, for the first time, the necessary properties of moons around free-flying planets have been determined in a new interdisciplinary collaboration to store liquid water long enough and thus make life possible.
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Artist’s view of Exomond with liquid water. |
Liquid water is essential for the emergence of life on Earth. Although only one planet is known to have formed where life originated, the scientific community has assumed that the presence of liquid water played a major role in the chemical evolution that could lead to the development of life elsewhere. Inside and outside our solar system, the habitable zone defines the annular region around the central star that is neither too hot nor too cold for liquid water on a planet.
Moons can also be habitable – even if they are planets outside the habitable zone. However, to do this, they must have a heat source other than stellar heat, such as shifting tidal forces. For example, thanks to tidal heating, there is an ocean of liquid water under the icy crust of Saturn’s moon Enceladus.
The discovery of dozens of free-flying planets (FFP) in our galaxy has changed understanding of the early evolution of planetary systems and theories of planet formation. These lonely wanderers may have been kicked out of their planetary system due to dynamic instability, and thus no longer have a host star. However, if they had a moon in a narrow orbit, they could gravitationally trap it to themselves. This works best for Jupiter-like planets that have Earth-sized moons. This creates unexpected new places where life can form.
In previous studies of liquid water on moons of starless planets, researchers from the ORIGINS group showed that Earth-sized moons around Jupiter-like planets may indeed contain liquid water. The results show that the amount of water that could be on the surface of the Moon is only a small fraction of the total volume of all of Earth’s oceans, but it is still a hundred times the water content of Earth’s atmosphere. This amount is enough to trigger chemical processes that can lead to life. Local dry-wet cycles (evaporation and condensation), as ASAL scientists recently demonstrated in a study of the first steps of evolution, provide the necessary chemical complexity that can drive molecular assembly and polymerization of RNA.
The exomonds’ orbit around the FFP becomes less tilted and more circular with time. This reduces tidal forces and thus heating efficiency. In a unique collaboration, doctoral student Giulia Rocchetti (ESO, former MA student at LMU) under the supervision of ASAL scientists, Professor Barbara Ercolano (Astrophysics), Dr. Karan Molaverdikhani, dr. Tommaso Grassi (Astrochemistry) and Professor Dieter Braun (Biochemistry) have developed a new realistic model that can calculate the evolution of the moon’s orbit over long periods of time. This is a time scale of several billion years, and is necessary for the evolution of life.
“In this way, we found that exomondes with small orbital radii not only have the best chance of surviving the expulsion of their planet from their planetary system, but they also remain exomondes for a long time,” explains Roccetti. Additionally, the denser atmosphere favors the preservation of liquid water. So, in particular, an Earth-sized moon with a Venus-like atmosphere close to its planet is a candidate for a habitable world.
The team reports its findings in a special article published in the journal International Astrobiology Journal Will appear.
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